Almost 80% of aniline consumption in the U.S. is used to produce MDI, an intermediate chemical which in turn is used to produce polyurethanes. Aniline is also used to produce rubber processing chemicals, dyes and pigments, specialty fibers, pesticides, and a variety of other materials including pharmaceuticals and photochemicals.
Direct combination of ammonia and benzene to form aniline and hydrogen is disfavored thermodynamically at reasonable temperatures and pressures leading to low equilibrium conversions for the reaction. This constraint can be overcome by conducting the process oxidatively according to equation 1, the method of our invention which we call oxidative ammination: ##STR1##
All current manufacturing processes to make aniline use either nitrobenzene or phenol as the immediate precursor. Nitrobenzene is the most common feedstock for aniline; it is prepared, in turn, by a mixed acid nitration of benzene. Nitrobenzene may then be hydrogenated to aniline in high yield either in liquid or vapor phase over catalysts containing Cu, Ni, or Pt. The use of corrosive acids and environmental concerns over the acid sludges generated are major deficiencies of the process to make aniline starting from benzene.
Only one U.S. manufacturer, Aristech, used the Halcon/Scientific Design process of ammonolysis of phenol, phenol being manufactured from cumene precursor. The gas phase ammonolysis of phenol may be carried out using oxide mixtures of Mg, B, and Ti on alumina or zeolitic supports combined with cocatalysts such as V or W salts. At large excesses of ammonia, phenol conversions of up to 98% can be obtained with a selectivity to aniline about 95%.
Equations (2) and (3) describe these processes: ##STR2##
As described above, an alternative to the present invention is the production of aniline from benzene by first forming phenol, and then using known technology to convert phenol to aniline in a second step. This route is not cost competitive, in most cases, with routes through nitrobenzene intermediate to aniline from benzene, because of the high cost of phenol. However, we have found that the catalysts disclosed herein are also useful in the direct formation of phenol from benzene in one step, thereby avoiding the using cumene intermediate and facilitating a potentially cheap route to phenol. With the availability of inexpensive phenol, a cheaper route to aniline, in turn, also could be realized that might compete more effectively with the usual nitrobenzene route to aniline. An object of the present invention is to provide an alternative route through a one-step process from benzene to aniline without the need to isolate phenol as an intermediate. That process is the focus of this invention.
Competitive direct synthesis of aniline from benzene and ammonia was patented by T. W. Del Pesco of the DuPont Company T. W. DelPesco, U.S. Pat. No. 4,031,106 (1977)!. The DuPont technology uses a Ni/NiO/ZrO.sub.2 "cataloreactant" which is reduced during each run, but which can be regenerated by a separate reoxidation. Although 12% yield of aniline is realized from benzene and ammonia with this technology, 7000 psi pressure is required to conduct the synthesis. This high pressure presents a formidable barrier to commercialization.
Mitsui Toatsu researchers have reported F. Matsuda and K. Kato, Japan Kokai J02115138-A (1990)! that by using an NH.sub.3 /H.sub.3 O mixture, it is possible to convert benzene to a mixture of phenol and aniline (1.9% yield of aniline) over a Cu.sub.3 (PO.sub.4).sub.2 /Ca.sub.3 (PO.sub.4).sub.2 catalyst at 300.degree.-500.degree. C. It is not clear how much oxygen is used in this work, but the reaction appears to be equilibrium limited without oxygen addition.
A. Heumann, K. J. Jens, and M. Reglier, "Palladium Complex Catalyzed Oxidation Reactions", K. D. Karlin, ed., Progress in Inorganic Chemistry, 42, New York, 1994, pp. 539-541, have described Pd nitro catalysts for alkene oxidation, but not for aromatic oxidation or ammination.
An advantage of the present invention is to provide a method for the oxidative ammination of aromatic compounds which does not require the added reagent and engineering costs and operational risks associated with the use of coreductants such as hydrogen or carbon monoxide.